1. Field of the Invention
The present invention relates to an aberration correction system for use in a transmission electron microscope and, more particularly, to an aberration correction system using three stages of multipole elements each producing a field of 3-fold symmetry.
2. Description of Related Art
One of the factors that limit the spatial resolution of an electron microscope is a variety of aberrations in the optical system. Especially, spherical aberration, which is one of such aberration, limits the spatial resolution because an axisymmetric lens always has a positive spherical aberration coefficient. This creates an intrinsic problem.
This problem has been dealt with in A. V. Crewe and D. Kopf, Optik, Vol. 55 (1980), pp. 1-10, where a result of a theoretical analysis has been shown. That is, a single stage hexapole element having a thickness along the optical axis has a negative spherical aberration coefficient. This suggests that spherical aberration can be reduced by introducing a hexapole element into the optical system. Subsequently, it has been pointed out that if only the single stage hexapole element is used, a second-order aberration occurs. Accordingly, incorporating a single stage hexapole element in a transmission electron microscope results in low usefulness. However, the fact that a hexapole element produces a negative spherical aberration coefficient is very useful to correction of spherical aberration. Techniques for reducing spherical aberration using hexapole elements have been improved further.
An example in which an aberration correction system equipped with a hexapole element having a negative spherical aberration and a thickness along the optical axis is applied to a transmission electron microscope is proposed in H. Rose, Optik, Vol. 85 (1990), pp. 19-24. This aberration correction system has a first transfer lens, a first hexapole element, a second transfer lens, and a second hexapole element arranged in turn. In this system, each transfer lens has two axisymmetric lenses.
An aberration correction system having two stages of multipole elements each having a thickness along the optical axis is shown in JP-A-2003-92078. This system has two stages of multipole elements (e.g., hexapole elements) and a transfer lens interposed between them. Each multipole element produces a field of 3-fold symmetry, generating a 3-fold astigmatism and a negative spherical aberration.
In the system of the above-cited JP-A-2003-92078, the rear stage of multipole element operates to cancel out the 3-fold astigmatism produced by the front stage multipole element and, therefore, the whole optical system produces a negative spherical aberration. Consequently, where an axisymmetric lens (e.g., an objective lens) producing a positive spherical aberration is disposed ahead of or behind the system, the spherical aberration in the whole optical system is reduced.
However, the above-described aberration-correcting techniques correct aberrations only up to the fourth order and cannot achieve complete correction of higher-order aberrations. For example, fifth-order spherical aberration can be corrected by optically controlling the distance between the objective lens and the aberration corrector but astigmatism of the same order (i.e., 6-fold astigmatism) cannot be corrected. Because this is a factor limiting aberration correction, it cannot be expected that the spatial resolution will be improved further.
An actual multipole element has a finite thickness along the optical axis. Where this multipole element produces a magnetic or electric field with 3-fold symmetry, if the spherical aberration is corrected by the multipole element, higher-order aberrations dependent on the thickness are induced. Furthermore, the combination of the two stages produces higher-order aberrations. Consequently, the range of incident angles of the electron beam that can be aberration-corrected is limited. This limitation makes it difficult to reduce diffraction aberration.
This limitation to the angles is further described by referring to the Ronchigram of FIG. 7. This diagram is obtained when an electron beam passing through two stages of multipole elements is corrected for aberrations, each of the multipole elements producing a magnetic field of 3-fold symmetry with respect to the optical axis. A low-contrast region appearing in the center of the diagram corresponds to the angle of incidence of the electron beam on each multipole element, the beam having been appropriately corrected for aberrations. Where a maximum value of the angle of incidence is roughly described, a maximum circle centered at the central point of the region and including only the region is fitted. The angle of incidence of the electron beam is computed from the radius of the circle. It can be seen from the diagram of FIG. 7 that the maximum incident angle of the electron beam that has been appropriately corrected for aberrations is about 50 mrad.
However, where regions located around the circle are noticed, one can observe that a region where an amorphous image is seen is hexagonal, because the fifth-order aberration, or the sixth-order astigmatism, is left as a residual aberration. In the case of the multipole elements producing the diagram of FIG. 7, the angle of incidence of the electron beam that can be corrected for aberrations is 50 mrad at maximum. It is difficult to appropriately correct the electron beam having a greater angle of incidence for aberrations. Accordingly, if one tries to reduce diffraction aberration, the spatial resolution is limited due to the limitation on the angle of incidence.
Higher-order aberrations (6-fold astigmatisms) produced from multipole elements that generate fields of 3-fold symmetry is induced because the magnetic or electric fields are distributed in directions to cancel out their mutual astigmatisms of 3-fold symmetry. That is, if multipole elements are rotated relative to each other such that each multipole element is rotated through 60° or 180° relative to the magnetic or electric field as in the prior art, higher-order aberrations are produced.